Super absorbent polymer

ABSTRACT

The present invention relates to a superabsorbent polymer exhibiting excellent heat stability.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase entry under 35 U.S.C. § 371 ofInternational Application No. PCT/KR2017/000157, filed Jan. 5, 2017,which claims priority to Korean Patent Application No. 10-2016-0135841,filed Oct. 19, 2016, Korean Patent Application No. 10-2016-0136727,filed Oct. 20, 2016, and Korean Patent Application No. 10-2016-0179499,filed Dec. 26, 2016, the disclosures of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION (a) Field of the Invention

The present invention relates to superabsorbent polymer exhibitingexcellent heat stability.

(b) Description of the Related Art

Super absorbent polymer (SAP) is synthetic polymer material that canabsorb moisture of 500 to 1000 times of self-weight, and is also nameddifferently as super absorbency material (SAM), absorbent gel material(AGM), etc. according to developing companies. The superabsorbentpolymer began to be commercialized as sanitary items, and currently, itis being widely used as hygienic goods such as a disposable diaper andso on, water-holding material for soil, water stop material for civilengineering and architecture, sheets for raising seedling, freshnesspreservatives in the field of food circulation, fomentation material,etc.

In most cases, such superabsorbent polymer is being widely used in thefield of hygienic goods such as a diaper or sanitary pad, etc., and forsuch use, it is required to exhibit high absorption power to moisture,etc., and the absorbed moisture should not escape even under externalpressure, and besides, it should properly maintain the shape even whenit absorbs water and the volume is expanded (swollen), thus exhibitingexcellent permeability.

However, it is known that centrifuge retention capacity (CRC) whichshows the basic absorption power and water retention capacity of thesuperabsorbent polymer, and absorbency under load (AUL) which shows theproperty of fully retaining absorbed moisture even under externalpressure, are difficult to simultaneously improve. If the overallcrosslinking density of the superabsorbent polymer is controlled to below, although centrifuge retention capacity may relatively increase, thecrosslink structure may be loosened and gel strength may decrease, andthus, absorbency under load may be deteriorated. To the contrary, if thecrosslinking density is controlled to be high so as to improveabsorbency under load, it may be difficult to absorb moisture betweenthe dense crosslink structures, and thus, basic centrifuge retentioncapacity may be deteriorated.

For the above explained reason, there was a limit to the provision ofsuperabsorbent polymer with simultaneously improved centrifuge retentionproperty and absorbency under load. In order to overcome this, therehave been various attempts to improve these properties together bycontrolling the kind or use amount of an internal crosslinking agent ora surface crosslinking agent, but such attempts have reached the limit.

And, during the production of hygienic goods such as diapers or femalesanitary pads, etc., superabsorbent polymer is inevitably exposed tohigh temperature. Thus, the superabsorbent polymer applied to hygienicgoods exhibits deteriorated absorption properties compared to thesuperabsorbent polymer immediately after synthesis. Such absorptionproperty deterioration of the superabsorbent polymer is directlyconnected to property deterioration of hygienic goods, and thus, thereis a continued demand for the development of technology enabling thepreparation of superabsorbent polymer with excellent heat stability.

SUMMARY OF THE INVENTION

The present invention relates to superabsorbent polymer exhibitingexcellent heat stability.

According to one embodiment of the present invention, superabsorbentpolymer comprising: a base resin powder comprising crosslinked polymerformed by the crosslinking polymerization of water soluble ethylenicallyunsaturated monomers having acid groups of which at least a part areneutralized, in the presence of an internal crosslinking agentcomprising a compound represented by the following Chemical Formula 1 inthe content of 80 wt % or more, based on the total weight of theinternal crosslinking agent; and a surface crosslink layer on the baseresin powder, formed by the additional crosslinking of the crosslinkedpolymer, wherein absorption property change rate at high temperature (Y)calculated by the following Calculation Formula 1 is −8% to 8%, isprovided:

in the Chemical Formula 1, R¹ is a divalent organic group derived fromC1-10 alkane, and R² is hydrogen or a methyl group,Y={[P ₁ −P ₂]/P ₁}*100  [Calculation Formula 1]

in the Calculation Formula 1,

Y is absorption property change rate at high temperature,

P₁ the sum of centrifuge retention capacity (CRC) of the superabsorbentpolymer to a saline solution, and absorbency under load (AUL) of 0.7 psiof the superabsorbent polymer to a saline solution, measured beforeexposed to high temperature, and

P₂ is the sum of CRC and AUL of the superabsorbent polymer, measuredafter leaving the superabsorbent polymer in an oven of 185° C. for 60minutes.

The P₁ of the superabsorbent polymer calculated by the CalculationFormula 1 may be 60 to 85 g/g. And, the P₂ of the superabsorbent polymercalculated by the Calculation Formula 1 may be 55 to 85 g/g.

In the Chemical Formula 1, R¹ may be methane-1,1-diyl, propane-1,3-diyl,propane-1,2-diyl, propane-1,1-diyl, n-butane-1,4-diyl,n-butane-1,3-diyl, n-butane-1,2-diyl, n-butane-1,1-diyl,2-methylpropane-1,3-diyl, 2-methylpropane-1,2-diyl,2-methylpropane-1,1-diyl, 2-methylbutane-1,4-diyl,2-methylbutane-2,4-diyl, 2-methylbutane-3,4-diyl,2-metylbutane-4,4-diyl, 2-methylbutane-1,3-diyl,2-methylbutane-1,2-diyl, 2-methylbutane-1,1-diyl or2-methylbutane-2,3-diyl. More specifically, in the Chemical Formula 1,R¹ may be propane-1,3-diyl or propane-1,2-diyl.

The base resin powder may comprise crosslinked polymer formed by thecrosslinking polymerization of water soluble ethylenically unsaturatedmonomers having acid groups of which at least a part are neutralized, inthe presence of an internal crosslinking agent comprising a compoundrepresented by the following Chemical Formula 1 in the content of 80 wt% or more, based on the total weight of the internal crosslinking agent,and inorganic material.

The inorganic material may be montmorillonite, saponite, nontronite,laponite, beidelite, hectorite, sauconite, stevensite, vermiculite,volkonskoite, magadite, medmontite, kenyaite, kaolin mineral, serpentinemineral, mica mineral, chlorite mineral, sepolite, palygorskite, bauxitesilica, alumina, titania or a mixture thereof.

Meanwhile, the superabsorbent polymer may comprise base resin powdercomprising crosslinked polymer formed by the crosslinking polymerizationof water soluble ethylenically unsaturated monomers having acid groupsof which at least a part are neutralized, in the presence of an internalcrosslinking agent comprising a compound represented by the followingChemical Formula 1 in the content of 80 wt % or more, based on the totalweight of the internal crosslinking agent; and a surface crosslink layeron the base resin powder, formed by the additional crosslinking of thecrosslinked polymer, wherein absorption property change rate at hightemperature (Y) calculated by the following Calculation Formula 1 may be−8% to 8%, and P₁ in the Calculation Formula 1 may be 60 to 85 g//g:

According to one embodiment of the present invention, unlike theexisting knowledge that centrifuge retention capacity and absorbencyunder load are inversely proportional to each other, superabsorbentpolymer exhibiting excellent properties with simultaneously improvedcentrifuge retention capacity and absorbency under load may be provided.And, the superabsorbent polymer may maintain excellent absorptionproperty even if exposed to high temperature, due to excellent heatstability. Thus, using the superabsorbent polymer, problems of theexisting superabsorbent polymer and technical requirement of the art canbe fundamentally solved, and various hygienic goods, etc. exhibitingmore excellent properties can be provided.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, superabsorbent polymer and a method for preparing the sameaccording to specific embodiments of the invention will be explained.

According to one embodiment of the present invention, superabsorbentpolymer comprising: a base resin powder comprising crosslinked polymerformed by the crosslinking polymerization of water soluble ethylenicallyunsaturated monomers having acid groups of which at least a part areneutralized, in the presence of an internal crosslinking agentcomprising a compound represented by the following Chemical Formula 1 inthe content of 80 wt % or more, based on the total weight of theinternal crosslinking agent; and a surface crosslink layer on the baseresin powder, formed by the additional crosslinking of the crosslinkedpolymer, wherein absorption property change rate (Y) at high temperaturecalculated by the following Calculation Formula 1 is −8% to 8%, isprovided:

in the Chemical Formula 1, R¹ is a divalent organic group derived fromC1-10 alkane, and R² is hydrogen or a methyl group,Y={[P ₁ −P ₂]/P ₁}*100  [Calculation Formula 1]

in the Calculation Formula 1,

Y is absorption property change rate at high temperature,

P₁ the sum of centrifuge retention capacity (CRC) of the superabsorbentpolymer to a saline solution, and absorbency under load (AUL) of 0.7 psiof the superabsorbent polymer to a saline solution, measured beforeexposed to high temperature, and

P₂ is the sum of CRC and AUL of the superabsorbent polymer, measuredafter leaving the superabsorbent polymer in an oven of 185° C. for 60minutes.

In general, if superabsorbent polymer is exposed to high temperature,the absorption property is deteriorated due to deformation by heat. Asthe result of experiments, the present inventors confirmed that ifsuperabsorbent polymer is prepared in the presence of a specific contentof a heat degradable internal crosslinking agent, even if thesuperabsorbent polymer is exposed to high temperature, the absorptionproperty may be maintained at excellent level. Such superabsorbentpolymer exhibiting excellent heat stability exhibits absorption propertychange rate at high temperature, calculated by the above CalculationFormula 1, of −8% to 8%.

Particularly, the superabsorbent polymer according to one embodiment mayexhibit absorption property change rate at high temperature, calculatedby the above Calculation Formula 1, of −5% to 7%, −3% to 6%, or −3% to4%. For a specific method of measuring absorption property change rateat high temperature, the contents of Experimental Examples describedbelow may be referred to.

The superabsorbent polymer exhibiting absorption property change rate athigh temperature, calculated by the Calculation Formula 1, of −8% to 8%,according to one embodiment, may have both improved centrifuge retentioncapacity and absorbency under load, thus exhibiting excellentproperties, unlike the existing knowledge that centrifuge retentioncapacity and absorbency under load are inversely proportional to eachother. Thus, in the Calculation Formula 1, P₁ may be 60 to 85 g/g, 62 to85 g/g, 64 to 85 g/g, 66 to 85 g/g, 68 to 85 g/g or 70 to 85 g/g, andthe P₂ may be 55 to 85 g/g, 60 to 85 g/g, 62 to 85 g/g, 65 to 85 g/g or70 to 85 g/g.

The superabsorbent polymer according to one embodiment may comprise baseresin powder comprising crosslinked polymer formed by the crosslinkingpolymerization of water soluble ethylenically unsaturated monomershaving acid groups of which at least a part are neutralized, in thepresence of an internal crosslinking agent comprising a compoundrepresented by the following Chemical Formula 1 in the content of 80 wt% or more, based on the total weight of the internal crosslinking agent;and a surface crosslink layer on the base resin powder, formed by theadditional crosslinking of the crosslinked polymer.

in the Chemical Formula 1, R¹ is a divalent organic group derived fromC1-10 alkane, and R² is hydrogen or a methyl group.

The compound represented by the following Chemical Formula 1 is a heatdegradable internal crosslinking agent, and the internal crosslinkstructure derived from the compound of the Chemical Formula 1 may bedegraded by heat. Thus, if water soluble ethylenically unsaturatedmonomers are crosslinked in the presence of the compound of the ChemicalFormula 1, crosslinked polymer into which a heat degradable internalcrosslink structure is introduced, may be provided. Thereafter, if suchcrosslinked polymer is subjected to subsequent high temperature process,at least a part of the crosslink structure derived from the compound ofthe Chemical Formula 1 in the crosslinked polymer may be degraded. Thus,the internal crosslinking density of the crosslinked polymer maydecrease. To the contrary, since the surface of the crosslinked polymeris additionally crosslinked by the surface crosslinking agent, theexternal crosslinking density may increase. Thus, if crosslinkingpolymerization is progressed using the compound of the Chemical Formula1 and subsequent process is conducted, the internal crosslink structureof the crosslinked polymer may be degraded, and the surface of thecrosslinked polymer may be additionally crosslinked, and thus,superabsorbent polymer wherein a crosslinking density increases from theinside toward the outside, may be obtained.

Thus prepared superabsorbent polymer may have more decreased internalcrosslinking density than the base resin of the existing superabsorbentpolymer. Thus, the superabsorbent polymer may exhibit relativelyimproved centrifuge retention capacity, compared to the existingsuperabsorbent polymer. And, the superabsorbent polymer may have thickersurface crosslink layer than the existing superabsorbent polymer,because surface crosslinking is progressed before or during thedegradation of internal crosslink. Thus, the superabsorbent polymer mayexhibit excellent absorbency under load. Therefore, since thecrosslinking density of the superabsorbent polymer of one embodimentincreases from the inside toward the outside, unlike the existingknowledge that centrifuge retention capacity and absorbency under loadare inversely proportional to each other, both centrifuge retentioncapacity and absorbency under load are improved, thus simultaneouslyexhibiting excellent properties.

And, due to the heat degradable internal crosslink structure remaininginside, the superabsorbent polymer may maintain excellent absorptionproperty, even if exposed to high temperature. Thus, the superabsorbentpolymer has a very small value of absorption property change rate athigh temperature, calculated by the above Calculation Formula 1.Consequently, the superabsorbent polymer of one embodiment canfundamentally solve the problems of the existing superabsorbent polymerand technical requirement of the art, and exhibit more excellentabsorption properties and heat stability.

Hereinafter, a method for preparing superabsorbent polymer according toone embodiment of the invention will be explained in detail.

The superabsorbent polymer according to one embodiment may be preparedby the steps of: conducting crosslinking polymerization of water solubleethylenically unsaturated monomers having acid groups of which at leasta part are neutralized, in the presence of an internal crosslinkingagent comprising a compound represented by the following ChemicalFormula 1 in the content of 80 wt % or more, based on the total weightof the internal crosslinking agent, to form hydrogel polymer; drying thehydrogel polymer to form base resin powder; and additionallycrosslinking the surface of the base resin in the presence of a surfacecrosslinking agent to form a surface crosslink layer.

in the Chemical Formula 1, R¹ is a divalent organic group derived fromC1-10 alkane, and R² is hydrogen or a methyl group.

In the step of forming hydrogel polymer, a monomer mixture comprisingwater soluble ethylenically unsaturated monomers having acid groups ofwhich at least a part are neutralized, and an internal crosslinkingagent, is subjected to crosslinking polymerization to form hydrogelpolymer.

The water-soluble ethylenically unsaturated monomers may include one ormore selected from the group consisting of anionic monomers and saltsthereof such as (meth)acrylic acid, maleic acid, maleic anhydride,fumaric acid, crotonic acid, itaconic acid, sorbic acid, vinylphosphinic acid, vinyl sulfonic acid, allyl sulfonic acid,2-(meth)acryloylethane sulfonic acid, 2-(meth)acryloyloxy ethanesulfonic acid, 2-(meth)acryloyl propane sulfonic acid, or2-(meth)acrylamido-2-methyl propane sulfonice acid; non-ionichydrophilic group containing monomers such as (meth)acrylamide,N-substituted (meth)acrylate, 2-hydroxyethyl (meth)acrylate,2-hydroxypropyl (meth)acrylate, methoxy polyethylene glycol(meth)acrylate, or polyethylene glycol (meth)acrylate; and amino groupcontaining unsaturated monomers such as (N,N)-dimethylaminoethyl(meth)acrylate, (N,N)-dimethylaminopropyl (meth)acrylamide, andquarternarized products thereof.

Throughout the specification, the term ‘water soluble ethylenicallyunsaturated monomers having acid groups of which at least a part areneutralized’ means that the water soluble ethylenically unsaturatedmonomers include monomers having acidic groups, and at least a part ofthe acidic groups of the monomers having acidic groups are neutralized.

Particularly, at least a part of the water soluble ethylenicallyunsaturated monomers may consist of monomers (the salts of anionicmonomers) in which the acidic groups included in the anionic monomersare neutralized.

More specifically, as the water soluble ethylenically unsaturatedmonomers, acrylic acid or salts thereof may be used, and in case acrylicacid is used, at least a part thereof may be neutralized. Due to the useof such monomers, superabsorbent polymer having more excellentproperties can be prepared. For example, in case an alkali metal salt ofacrylic acid is used as the water soluble ethylenically unsaturatedmonomers, acrylic acid may be neutralized with a neutralization agentsuch as caustic soda (NaOH) before use. Here, the neutralization degreeof the acrylic acid may be controlled to about 50 to 95 mol %, or about60 to 85 mol %, and within such range, superabsorbent polymer withexcellent centrifuge retention capacity can be provided without concernof precipitation during neutralization.

In the monomer mixture comprising the water-soluble ethylenicallyunsaturated monomers, the concentration of the water-solubleethylenically unsaturated monomers may be controlled to about 20 toabout 60 wt %, or about 25 to about 50 wt %, based on the monomermixture comprising raw materials described below, a polymerizationinitiator and a solvent, etc., and may be appropriately controlledconsidering polymerization time and reaction conditions, etc. However,if the concentration of the monomers becomes too low, yield ofsuperabsorbent polymer may decrease, thus causing economical problems,and if the concentration becomes too high, process problems may begenerated such as precipitation of a part of the monomers or lowgrinding efficiency of polymerized hydrogel polymer, etc., and theproperties of superabsorbent polymer may be deteriorated.

As the internal crosslinking agent, a compound represented by theChemical Formula 1 is used so as to introduce an internal crosslinkstructure that can be degraded by heat into the crosslinked polymer ofwater soluble ethylenically unsaturated monomers.

In the Chemical Formula 1, R¹ is a divalent organic group derived fromC1-10 alkane, and R² is hydrogen or a methyl group, as defined above,Here, the alkane may be linear, branched or cyclic alkane, and thedivalent organic group derived from such alkane may be a divalentorganic group wherein two hydrogen atoms are removed from one carbon, ora divalent organic group wherein each one hydrogen is removed fromdifferent carbon atoms. Specifically, R¹ may be methane-1,1-diyl,ethane-1,2-diyl, ethane-1,1-diyl, propane-1,3-diyl, propane-1,2-diyl,propane-1,1-diyl, n-butane-1,4-diyl, n-butane-1,3-diyl,n-butane-1,2-diyl, n-butane-1,1-diyl, 2-methylpropane-1,3-diyl,2-methylpropane-1,2-diyl, 2-methylpropane-1,1-diyl,2-methylbutane-1,4-diyl, 2-methylbutane-2,4-diyl,2-methylbutane-3,4-diyl, 2-metylbutane-4,4-diyl,2-methylbutane-1,3-diyl, 2-methylbutane-1,2-diyl,2-methylbutane-1,1-diyl or 2-methylbutane-2,3-diyl.

Among them, R¹ in the Chemical Formula 1, may be methane-1,1-diyl,propane-1,3-diyl, propane-1,2-diyl, propane-1,1-diyl, n-butane-1,4-diyl,n-butane-1,3-diyl, n-butane-1,2-diyl, n-butane-1,1-diyl,2-methylpropane-1,3-diyl, 2-methylpropane-1,2-diyl,2-methylpropane-1,1-diyl, 2-methylbutane-1,4-diyl,2-methylbutane-2,4-diyl, 2-methylbutane-3,4-diyl,2-metylbutane-4,4-diyl, 2-methylbutane-1,3-diyl,2-methylbutane-1,2-diyl, 2-methylbutane-1,1-diyl or2-methylbutane-2,3-diyl.

Specifically, R¹ in the Chemical Formula 1, may be methane-1,1-diyl,propane-1,3-diyl, or propane-1,2-diyl. More specifically, R¹ in theChemical Formula 1, may be propane-1,3-diyl, or propane-1,2-diyl.

The compound of the Chemical Formula 1 wherein R¹ is one of the abovelisted divalent organic groups may provide an internal crosslinkstructure of which degradability by heat energy can be easilycontrolled, and it may not generate by-products or water-solublecomponents that change the properties of superabsorbent polymer afterdegradation.

The internal crosslinking agent may comprise the compound represented bythe Chemical Formula 1, in the content of 80 wt % to 100 wt %, 85 wt %to 100 wt %, 90 to 100 wt %, 95 to 100 wt % or 100 wt %, based on thetotal weight of the internal crosslinking agent. Within such ranges, thesuperabsorbent polymer according to one embodiment of the presentinvention may have crosslinking density gradient of the aimed level andheat stability. For example, the internal crosslinking agent may consistof the compound represented by the Chemical Formula 1. Namely, in casethe entire amount of the internal crosslinking agent is the compoundrepresented by the Chemical Formula 1, excellent heat stability may besecured as well as excellent absorption properties.

If the compound represented by the Chemical Formula 1 is used in thecontent of 80 wt % or more and less than 100 wt %, based on the totalweight of the internal crosslinking agent, the internal crosslinkingagent may comprise the remaining amount of the existing internalcrosslinking agents. As the existing internal crosslinking agent,compounds comprising two or more crosslinkable functional groups in themolecule may be used. The existing internal crosslinking agent maycomprise a carbon-carbon double bond as the crosslinkable functionalgroup for the smooth crosslinking polymerization of the above explainedwater soluble ethylenically unsaturated monomers. Specifically, as theexisting internal crosslinking agent, one or more selected from thegroup consisting of polyethyleneglycol diacrylate (PEGDA), glycerindiacrylate, glycerin triacrylate, non-modified or ethoxylatedtrimethylolpropane, triacrylate (TMPTA), hexanediol diacrylate, allyl(meth)acrylate and triethyleneglycol diacrylate may be used.

The internal crosslinking agent may be used in the content of 0.01 to 5parts by weight, 0.01 to 3 parts by weight, 0.1 to 3 parts by weight, or0.2 to 1.5 parts by weight, based on 100 parts by weight of the watersoluble ethylenically unsaturated monomers. Here, the content of thewater soluble ethylenically unsaturated monomers is based on the weightof the water soluble ethylenically unsaturated monomers before theacidic groups of the monomers having acidic groups included in the watersoluble ethylenically unsaturated monomers are neutralized. For example,in case the water soluble ethylenically unsaturated monomers includeacrylic acid, the content of the internal crosslinking agent may becontrolled on the basis of the weight of the monomer before acrylic acidis neutralized.

And, the internal crosslinking agent may be used in an appropriateconcentration, based on the monomer mixture.

The internal crosslinking agent may be used within the above explainedranges to provide superabsorbent polymer that has a suitablecrosslinking density gradient, and thus, has both improved centrifugeretention capacity and absorbency under load, and exhibits excellentheat stability.

Meanwhile, the monomer mixture may further comprise inorganic materialso as to prepare superabsorbent polymer exhibiting excellent absorptionproperties. Thus, the super absorbent polymer may comprise base resinpowder comprising crosslinked polymer formed by the crosslinkingpolymerization of water soluble ethylenically unsaturated monomershaving acid groups of which at least a part are neutralized, in thepresence of an internal crosslinking agent comprising a compoundrepresented by the following Chemical Formula 1 in the content of 80 wt% or more, based on the total weight of the internal crosslinking agent,and inorganic material.

As the inorganic material, for example, montmorillonite, saponite,nontronite, laponite, beidelite, hectorite, sauconite, stevensite,vermiculite, volkonskoite, magadite, medmontite, kenyaite, kaolinmineral, serpentine mineral, mica mineral, chlorite mineral, sepolite,palygorskite, bauxite silica, alumina, titania or a mixture thereof maybe used.

Among them, laponite can effectively improve heat stability, centrifugeretention capacity and absorbency under load.

The organic material may be added in the amount of about 0.001 to 1.0 wt%, based on the monomer mixture, to realize excellent absorptionproperties.

And, the monomer mixture may further comprise a polymerization initiatorcommonly used in the preparation of superabsorbent polymer.

Specifically, the polymerization initiator may be appropriately selectedaccording to polymerization methods, a thermal polymerization initiatormay be used when a thermal polymerization method is used, aphotopolymerization initiator may be used when a photopolymerizationmethod is used, and both thermal polymerization initiator andphotopolymerization initiator may be used when a hybrid polymerizationmethod (method using both heat and light) is used. However, even in thecase of photopolymerization, since a certain amount of heat is generatedby UV irradiation, etc., and heat is generated to some degree accordingto the progression of an exothermic polymerization reaction, a thermalpolymerization initiator may be additionally included.

The photopolymerization initiator is not limited in terms of itsconstruction, as long as it is a compound capable of forming a radicalby light such as UV.

As the photopolymerization initiator, one or more selected from thegroup consisting of benzoin ether, dialkyl acetophenone, hydroxylalkylketone, phenyl glyoxylate, benzyl dimethyl Ketal, acyl phosphine,and α-aminoketone may be used. Specific example of the acyl phosphinemay include diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide,phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, ethyl(2,4,6-trimethylbenzoyl)phenylphosphinate, etc. More variousphotopolymerization initiators are described in Reinhold Schwalm, “UVCoatings: Basics, Recent Developments and New Application (Elsevier2007)”, page 115, and are not limited to the above described examples.

The photopolymerization initiator may be added in the concentration ofabout 0.0001 to about 2.0 wt %, based on the monomer composition. If theconcentration of the photopolymerization initiator is too low,polymerization speed may become slow, and if the concentration of thepolymerization initiator is too high, the molecular weight of thesuperabsorbent polymer may become small and the properties may becomenon-uniform.

And, as the thermal polymerization initiator, at least one selected fromthe group consisting of a persulfate initiator, an azo initiator,hydrogen peroxide, and ascorbic acid may be used. Specific examples ofthe persulfate initiator may include sodium persulfate (Na₂S₂O₈),potassium persulfate (K₂S₂O₈), ammonium persulfate ((NH₄)₂S₂O₈), etc.,and, specific examples of the azo initiator may include2,2-azobis(2-amidinopropane)dihydrochloride,2,2-azobis-(N,N-dimethylene)isobutyramidinedihydrochloride,2-(carbamoylazo)isobutyronitril,2,2-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride,4,4-azobis-(4-cyanovalericacid), etc. More various thermal initiatorsare described in “Principle of Polymerization (Wiley, 1981)”, Odian,page 203, and are not limited to the above described examples.

The thermal polymerization initiator may be included in theconcentration of about 0.001 to about 2.0 wt %, based on the monomercomposition. If the concentration of the thermal polymerizationinitiator is too low, additional thermal polymerization may hardlyoccur, and thus, the effect obtained by the addition of the thermalpolymerization initiator may be insignificant, and if the concentrationof the thermal polymerization initiator is too high, the molecularweight of the superabsorbent polymer may become small, and theproperties may become non-uniform.

The monomer mixture may further comprise additives such as a thickener,a plasticizer, a preservation stabilizer, an antioxidant, etc., asnecessary.

The above explained raw materials such as water soluble ethylenicallyunsaturated monomers, an internal crosslinking agent, inorganicmaterial, a polymerization initiator and additives may be prepared inthe form of a solution dissolved in a solvent.

Here, the solvent that can be used is not limited in terms of itsconstruction as long as it can dissolve or disperse the above explainedcomponents, and for example, one or more selected from water, ethanol,ethyleneglycol, diethyleneglycol, triethyleneglycol, 1,4-butanediol,propyleneglycol, ethyleneglycol monobutyl ether, propyleneglycolmonomethyl ether, propyleneglycol monomethyl ether acetate,methylethylketone, acetone, methylamylketone, cyclohexanone,cyclopentanone, diethyleneglycol monomethyl ether, diethyleneglycolethyl ether, toluene, xylene, butyrolactone, carbitol, methylcellosolveacetate and N,N-dimethylacetamide, etc. may be used alone or incombination.

The solvent may be included in the remaining amount excluding theabove-explained components, based on the total amount of the monomermixture.

Meanwhile, a method of forming hydrogel polymer by the thermalpolymerization, photopolymerization or hybrid polymerization of themonomer composition is not specifically limited in terms of itsconstruction, as long as it is a commonly used polymerization method.

Specifically, the polymerization method is largely classified intothermal polymerization and photopolymerization according to energysource. Commonly, thermal polymerization may be progressed in a reactorequipped with a stirring axis such as a kneader, and, in case thermalpolymerization is progressed, it may be progressed at a temperature ofabout 80° C. or more and less than about 110° C. so that the compoundrepresented by the Chemical Formula 1 may not be degraded by heat. Ameans to achieve the polymerization temperature of the above explainedrange is not specifically limited, and a heating medium may be suppliedto the reactor or a heat source may be directly supplied to heat. Thekinds of the heating medium that can be used may includetemperature-raised fluid such as steam, hot air, hot oil, etc., but arenot limited thereto, and may be appropriately selected considering themeans of the heating medium, temperature rise speed and targettemperature to be increased. Meanwhile, the heat source directlysupplied may include electric heating, gas heating, etc., but is notlimited thereto.

Meanwhile, photopolymerization may be progressed in a reactor equippedwith a movable conveyer belt, but the above explained polymerizationmethods are no more than examples, and the present invention is notlimited thereto.

For example, in case thermal polymerization is progressed by supplying aheating medium into a reactor equipped with a stirring axis such as akneader as explained above or heating the reactor, hydrogel polymerdischarged to the outlet of the reactor may be obtained. The hydrogelpolymer may be obtained in the size of a few centimeters to a fewmillimeters according to the shape of the stirring axis equipped in thereactor. Specifically, the size of obtained hydrogel polymer may varyaccording to the concentration of the introduced monomer mixture and theintroduction speed, etc.

And, in case photopolymerization is progressed in a reactor equippedwith a movable conveyer belt as explained above, the obtained hydrogelpolymer may be in the form of a sheet having the width of the belt.Here, the thickness of the polymer sheet may vary according to theconcentration of the introduced monomer mixture and the introductionspeed, but, commonly, a monomer mixture is preferably fed such thatpolymer in the form of a sheet having a thickness of about 0.5 cm toabout 10 cm may be obtained. In case a monomer mixture is fed such thatthe thickness of sheet-shaped polymer may be too thin, productionefficiency may be low, and if the thickness of the sheet-shaped polymeris greater than 10 cm, due to the too thick thickness, a polymerizationreaction may not uniformly occur throughout the whole thickness.

The polymerization time of the monomer mixture is not specificallylimited, and it may be controlled to about 30 seconds to 60 minutes.

Here, the moisture content of hydrogel polymer obtained by such a methodmay be about 30 to about 80 wt %. Throughout the specification, the“moisture content” is the content of moisture occupied based on thetotal weight of hydrogel polymer, and it means a value obtained bysubtracting the weight of polymer of a dry state from the weight ofhydrogel polymer. Specifically, it is defined as a value calculated bymeasuring the weight loss according to moisture evaporation in thepolymer while raising the temperature of polymer through infraredheating to dry. At this time, the drying condition is established suchthat the temperature is raised from room temperature to about 180° C.and then maintained at 180° C., and the total drying time is 20 minutesincluding a temperature raising step of 5 minutes.

In the step of forming base resin powder, the hydrogel polymer obtainedthrough the step of forming hydrogel polymer is dried to provide baseresin powder.

In the step of forming base resin powder, a coarse grinding process maybe included before drying the hydrogel polymer so as to increase dryingefficiency.

Here, grinders that can be used in the coarse grinding is not limited interms of the constructions, but specifically, one selected from thegroup consisting of a vertical pulverizer, a turbo cutter, a turbogrinder, a rotary cutter mill, a cutter mill, a disc mill, a shredcrusher, a crusher, a chopper, a disc cutter may be used, but is notlimited thereto.

Through the coarse grinding step, the particle diameter of the hydrogelpolymer may be controlled to about 0.1 to about 10 mm. Grinding to aparticle diameter of less than 0.1 mm would not be technically easy dueto the high moisture content of the hydrogel polymer, and may generateagglomeration between the ground particles. Meanwhile, if grinding to aparticle diameter greater than 10 mm, the effect of increasing theefficiency of the subsequent drying step may be insignificant.

The hydrogel polymer coarsely ground as explained above, or hydrogelpolymer immediately after polymerization that does not pass through thecoarse grinding step is dried, and the drying temperature may be about20° C. to about 250° C. If the drying temperature is less than about 20°C., a drying time may too lengthen, and the properties of the finallyprepared superabsorbent polymer may be deteriorated, and if the dryingtemperature is greater than about 250° C., only the surface of hydrogelpolymer may be dried, thus generating fine powder in the subsequentgrinding process, and the properties of the finally preparedsuperabsorbent polymer may be deteriorated. Preferably, the drying maybe progressed at a temperature of about 40 to 200° C., more preferablyat 110 to 200° C.

Particularly, if the drying temperature of the hydrogel polymer is about110° C. to about 200° C., at least a part of the crosslink structurederived from the compound represented by the Chemical Formula 1 may bedegraded by heat. As the result, the internal crosslinking density ofthe crosslinked polymer may decrease in the drying step. Suchcrosslinked polymer with decreased internal crosslinking density mayprovide superabsorbent polymer with remarkably improved centrifugeretention capacity, compared to the crosslinked polymer of whichinternal crosslinking density has not decreased.

Meanwhile, the drying may be progressed for 20 minutes to 120 minutesconsidering the process efficiency, etc. For example, the hydrogelpolymer may be dried for about 20 minutes to 100 minutes or about 30minutes to about 50 minutes, so that the internal crosslink structuremay be sufficiently degraded.

And, the drying method is not limited in terms of the construction aslong as it can be commonly used as a drying process of hydrogel polymer.Specifically, the drying step may be progressed by hot wind supply,infrared ray irradiation, ultrahigh frequency wave irradiation, or UVirradiation, etc. The polymer dried by such a method may exhibit amoisture content of about 0.1 to about 10 wt %.

The step of forming base resin powder may further comprise a step ofgrinding dried polymer obtained through the drying step.

The particle diameter of the polymer powder obtained after the grindingstep may be 150 μm to 850 μm. As a grinder for grinding to such aparticle diameter, specifically, a pin mill, a hammer mill, a screwmill, a roll mill, a disc mill, or a jog mill, etc. may be used, but thegrinder is not limited thereto.

And, the step of forming base resin powder may further comprise a stepof sieving ground polymer obtained through the grinding step. That is,in the step of forming base resin powder, the hydrogel polymer may bedried, ground and sieved to provide base resin powder.

After the grinding step, a step of sieving the polymer powder accordingto the particle diameter may be conducted so as to manage the propertiesof the finally productized superabsorbent polymer. It is appropriatethat base rein power and superabsorbent polymer obtained therefrom areprepared and provide with particle diameters of about 150 to 850 μm,through the processes of grinding and sieving, etc. More specifically,at least about 95% of the base resin powder and superabsorbent polymerobtained therefrom may have particle diameters of about 150 to 850 μm,and less than about 3 wt % thereof may be fine powder with particlediameters of less than about 150 μm

As explained, since the particle diameter distributions of the baseresin powder and superabsorbent polymer are controlled within preferableranges, the finally prepared superabsorbent polymer may exhibitexcellent absorption properties. Thus, in the sieving step, polymer withparticle diameters of about 150 to about 850 μm may be sieved, and onlythe polymer powders having such particle diameters may be subjected tosurface crosslinking and productized.

Meanwhile, after forming the base resin powder, in the presence of asurface crosslinking agent, the surface of the base resin powder may beadditionally crosslinked to form a surface crosslink layer, therebypreparing superabsorbent polymer.

As the surface crosslinking agent, any surface crosslinking agents usedfor the preparation of superabsorbent polymer before may be used withoutspecific limitations.

Specific examples thereof may include one or more polyols selected fromthe group consisting of ethyleneglycol, propylene glycol,1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, 1,2-hexanediol,1,3-hexanediol, 2-methyl-1,3-propanediol, 2,5-hexanediol,2-methyl-1,3-pentandiol, 2-methyl-2,4-pentanediol, tripropylene glycoland glycerol; one or more carbonate-based compounds selected from thegroup consisting of ethylene carbonate and propylene carbonate; epoxycompounds such as ethyleneglycol diglycidylether, etc.; oxazolinecompounds such as oxazolidinone, etc.; polyamine compounds; oxazolinecompounds; mono-, di-, or polyoxazolidinone compounds; or cyclic ureacompounds, etc.

Such a surface crosslinking agent may be used in the content of about0.01 to 3 parts by weight, based on 100 parts by weight of the baseresin powder. By controlling the content range of the surfacecrosslinking agent within the above explained range, superabsorbentpolymer exhibiting excellent absorption properties may be provided.

And, in the surface crosslinking process, in addition to the surfacecrosslinking agent, one or more inorganic materials selected from thegroup consisting of silica, clay, alumina, silica-alumina composite,titania, zinc oxide and aluminum sulfate may be further added to conducta surface crosslinking reaction. The inorganic material may be used inthe form of powder or liquid, and particularly, in the form of aluminapowder, silica-alumina powder, titania powder, or nano silica solution.And, the inorganic material may be used in the content of about 0.001 toabout 2 parts by weight, based on 100 parts by weight of the base resinpowder.

And, in the surface crosslinking process, instead of or in addition tothe inorganic material, multivalent metal cation may be added toprogress surface crosslinking, thereby optimizing the surface crosslinkstructure of superabsorbent polymer. It is predicted that such metalcation may form chelate with the carboxy group (COOH) of superabsorbentpolymer, thus further reducing the crosslinking distance.

And, the method of adding the surface crosslinking agent to base resinpowder is not limited in terms of its construction. For example, asurface crosslinking agent and base resin powder may be put in a reactorand mixed, a surface crosslinking agent may be sprayed to base resinpowder, or base resin powder and a surface crosslinking agent may becontinuously fed to a continuously operated mixed and mixed.

When the surface crosslinking agent is added, water and methanol may bemixed together and added. In case water and methanol are added, thesurface crosslinking agent may be uniformly dispersed in the base resinpower. Here, the contents of water and methanol added may beappropriately controlled so as to induce uniform dispersion of thesurface crosslinking agent, preventing the agglomeration of the baseresin powder, and optimizing the surfacepenetration depth of thecrosslinking agent.

By heating the base resin powder to which the surface crosslinking agentis added beyond a specific temperature, a surface crosslinking reactionmay be achieved. In such a surface crosslinking step, an internalcrosslink degradation reaction may be achieved simultaneously with thesurface crosslinking reaction. Thus, in the surface crosslinking step,at least a part of the crosslink structure derived from the compound ofthe Chemical Formula 1 in the base resin powder may be heat degraded,and thus, internal crosslinking density may decrease. And, due to theabove reactions, superabsorbent polymer with a crosslinking densitygradient increasing from the inside toward the outside may be prepared.

Particularly, in order to prepare superabsorbent polymer withsimultaneously improved centrifuge retention capacity and absorbencyunder load, the surface crosslinking reaction may be conducted at atemperature of about 110 to 200° C.

More specifically, the surface crosslinking conditions may include amaximum reaction temperature of about 160° C. or more, or about 180 to200° C., and a maintenance time at the maximum reaction temperature ofabout 20 minutes or more, or about 20 minutes to 2 hours. And, a timeduring which a temperature is increased from the temperature at thebeginning of the reaction, for example, about 110° C. or more, or about160 to 170° C., to the above maximum reaction temperature, may becontrolled to about 10 minutes or more, or about 10 minutes to 1 hour,and it was confirmed that superabsorbent polymer with both excellentcentrifuge retention capacity and absorbency under load may be preparedby satisfying the above explained surface crosslinking processconditions.

A temperature rise means for the surface crosslinking reaction is notspecifically limited. A heating medium may be supplied, or a heat sourcemay be directly supplied to heat. Here, the kinds of the heating mediumthat can be used may include temperature-increased fluid such as steam,hot air, hot oil, etc., but are not limited thereto, and may beappropriately selected considering the means of the heating medium,temperature rise speed and a temperature to be increased. Meanwhile, theheat source directly supplied may include electric heating, gas heating,etc., but is not limited thereto.

The superabsorbent polymer obtained by the above explained preparationmethod has a crosslinking density increasing from the inside toward theoutside, because a part of the heat-degradable internal crosslinkstructure is degraded in the subsequent high temperature process afterthe polymerization process, thereby exhibiting very excellent propertieswith simultaneously improved centrifuge retention capacity andabsorbency under load. And, the superabsorbent polymer may maintain theabsorption properties of the superabsorbent polymer, even if exposed tohigh temperature. Thus, the superabsorbent polymer, even if passedthrough a high temperature production process, can provide hygienicgoods such as a diaper, etc. with excellent absorption properties.

Hereinafter, the actions and the effects of the invention will beexplained in detail through the specific examples. However, theseexamples are presented only as the illustrations of the invention, andthe scope of the right of the invention is not limited thereby.

Example 1: Preparation of Superabsorbent Polymer

Into a glass reactor, 100 g of acrylic acid, 0.6 g of4-methylpentane-1,4-diyl diacrylate as an internal crosslinking agent,0.008 g of Irgacure TPO (diphenyl(2,4,6-trimethylbenzoyl)phosphineoxide), 0.18 g of laponite and 55 g of water were put. And, to the glassreactor, 123.5 g of 32 wt % caustic soda solution was slowly addeddropwise and mixed.

When adding the caustic soda solution dropwise, the temperature of themixed solution increased by neutralization heat, thus waited until themixed solution was cooled. When the temperature of the mixed solutionwas cooled to about 45° C., 0.2 g of sodium persulfate was added to themixed solution to prepare a monomer mixture.

On a conveyer belt with a width of 10 cm and a length of 2 m, rotatingat the velocity of 50 cm/min, the monomer mixture was fed at 500˜2000mL/min. And, simultaneously with the feeding of the monomer mixture, UVwas irradiated at the intensity of 10 mW/cm² to progress apolymerization reaction for 60 seconds.

And, the polymer obtained through the polymerization reaction was passedthrough a hole with a diameter of 10 mm to prepare as crumb using a meatchopper. Subsequently, the crumb was uniformly dried by flowing hot airof 185° C. from the lower part to the upper part for 20 minutes using anoven capable of transferring air volume upward and downward, and flowingit again from the upper part to the lower part for 20 minutes. The driedcrumb was ground with a grinder, and then, sieved to obtain base resinwith a size of 150 to 850 μm.

To 100 g of the above prepared base resin powder, a mixed solution of3.2 g of deionized water, 4.0 g of methanol, 0.088 g of ethylenecarbonate, and 0.01 g of silica (product name: REOLOSIL DM30S,manufacturing company: Tokuyama Corporation) was added, and mixed for 1minute, and then, surface crosslinking was conducted at 185° C. for 90minutes.

And, the obtained product was ground and sieved to obtain superabsorbentpolymer with a particle diameter of 150 to 850 μm.

Example 2: Preparation of Superabsorbent Polymer

Superabsorbent polymer was prepared by the same method as Example 1,except that 0.6 g of 2-methylpentane-2,4-diyl diacrylate was used as aninternal crosslinking agent, instead of 0.6 g of4-methylpentane-1,4-diyldiacrylate in Example 1.

Example 3: Preparation of Superabsorbent Polymer

Superabsorbent polymer was prepared by the same method as Example 1,except that 0.6 g of 2-methylpropane-1,2-diyl diacrylate was used as aninternal crosslinking agent, instead of 0.6 g of4-methylpentane-1,4-diyldiacrylate in Example 1.

Example 4: Preparation of Superabsorbent Polymer

Superabsorbent polymer was prepared by the same method as Example 1,except that 0.6 g of 2-methylbutane-2,4-diyl diacrylate was used as aninternal crosslinking agent, instead of 0.6 g of4-methylpentane-1,4-diyldiacrylate in Example 1.

Comparative Example 1: Preparation of Superabsorbent Polymer

Superabsorbent polymer was prepared by the same method as Example 1,except that 0.4 g of polyethyleneglycol diacrylate (PEGDA, weightaverage molecular weight 598 g/mol) was used as an internal crosslinkingagent, instead of 0.6 g of 4-methylpentane-1,4-diyl diacrylate inExample 1.

For reference, in case the content of polyethyleneglycol diacrylate wasadjusted to 0.6 g as Example 1, superabsorbent polymer with veryinferior centrifuge retention capacity was prepared, and thus, thecontent of polyethyleneglycol diacrylate was adjusted to 0.4 g, unlikeExample 1, to prepare superabsorbent polymer.

Comparative Example 2: Preparation of Superabsorbent Polymer

Superabsorbent polymer was prepared by the same method as Example 4,except that 0.4 g of polyethyleneglycol diacrylate (PEGDA, weightaverage molecular weight 598 g/mol) was used as an internal crosslinkingagent, instead of 0.6 g of 2-methylbutane-2,4-diyl diacrylate in Example4.

Comparative Example 3: Preparation of Superabsorbent Polymer

Superabsorbent polymer was prepared by the same method as Example 4,except that 0.6 g of polyethyleneglycol (PEG, weight average molecularweight 600 g/mol) was used as an internal crosslinking agent, instead of0.6 g of 2-methylbutane-2,4-diyldiacrylate in Example 4.

Experimental Example: Evaluation of the Properties of SuperabsorbentPolymer

The properties of the superabsorbent polymers prepared according toExamples and Comparative Examples were evaluated as follows, and shownin the following Table 1.

(1) Absorption Property Change Rate at High Temperature

The superabsorbent polymers prepared according to Examples andComparative Examples were exposed to high temperature, and then, changesin the absorption properties before and after exposure to hightemperature were measured.

Specifically, from the superabsorbent polymer prepared according toExamples and Comparative Examples, samples were taken and the centrifugeretention capacity (CRC, unit: g/g) to a saline solution and absorbencyunder load (AUL, unit: g/g) to a saline solution were measured. The CRCand AUL were measured according to (2) and (3) below.

Meanwhile, from the superabsorbent polymer prepared according toExamples and Comparative Examples, samples were taken and left in anoven of 185° C. for 60 minutes, and then, the CRC and AUL of the samplesexposed to high temperature were measured according to (2) and (3)below. And, using the obtained CRC and AUL values, absorption propertychange rate at high temperature was confirmed by the followingCalculation Formula 1.Y={[P ₁ −P ₂]/P ₁}*100  [Calculation Formula 1]

in the Calculation Formula 1,

Y is absorption property change rate at high temperature,

P₁ the sum of centrifuge retention capacity (CRC) of the superabsorbentpolymer to a saline solution, and absorbency under load (AUL) of 0.7 psiof the superabsorbent polymer to a saline solution, measured beforeexposed to high temperature, and

P₂ is the sum of CRC and AUL of the superabsorbent polymer, measuredafter leaving the superabsorbent polymer in an oven of 185° C. for 60minutes.

(2) Centrifuge Retention Capacity (CRC)

The centrifuge retention capacity (CRC) of the superabsorbent polymer toa saline solution was measured according to EDANA method NWSP 241.0.R2.

Specifically, among the superabsorbent polymer of which centrifugeretention capacity is to be measured, a sample with the particlediameter of 150 to 850 μm, which passes through a US standard 20 meshscreen, and remains on a US standard 100 mesh screen, was prepared.

And, W₀ (g, about 0.2 g) of the sample having a particle diameter of 150to 850 μm were uniformly put in an envelope made of non-woven fabric,and the envelope was sealed. And, the envelope was soaked in a 0.9 wt %sodium chloride aqueous solution (saline solution) at room temperature.After 30 minutes, the envelope was drained at 250 G for 3 minutes usinga centrifuge, and then, the mass W₂ (g) of the envelope was measured.And, after the same operation using an empty envelope without a sample,the mass W₁ (g) at that time was measured.

Using the obtained masses, CRC (g/g) was calculated according to thefollowing Calculation Formula 2.CRC(g/g)={[W ₂(g)−W ₁(g)]/W ₀(g)}−1  [Calculation Formula 2]

In the Calculation Formula,

W₀ (g) is the initial weight of the sample having a particle diameter of150 to 850 μm (g),

W₁ (g) the weight of an empty envelope made of nonwoven fabric, measuredafter the empty envelope without a sample was soaked in a salinesolution at room temperature for 30 minutes, and then, drained using acentrifuge at 250 G for 3 minutes, and

W₂ (g) is the weight of an envelope made of nonwoven fabric includingthe sample, measured after the envelope made of nonwoven fabricincluding the sample was soaked in a saline solution at room temperaturefor 30 minutes, and then, drained using a centrifuge at 250 G for 3minutes.

(3) Absorbency Under Load (AUL)

The absorbency under load (AUL) of 0.7 psi of the superabsorbent polymerto a saline solution was measured according to EDANA method NWSP242.0.R2.

Specifically, a 400 mesh screen made of stainless was installed on thebottom of a plastic cylinder with an inner diameter of 25 mm. Under theconditions of room temperature and relative humidity of 50%, W₀ (g, 0.16g) of superabsorbent polymer of which absorbency under load is to bemeasured were uniformly scattered on the screen. Subsequently, a pistonthat can uniformly give a load of 4.8 kPa (0.7 psi) was added on thesuperabsorbent polymer. Here, as the piston, a piston having an outerdiameter slightly smaller than 25 mm was used such that there was no gapwith the inner wall of the cylinder, and the movement upward anddownward was not hindered. At this time, the weight W₃ (g) of theapparatus was measured.

Subsequently, on the inner side of a petri dish with a diameter of 150mm, a glass filter with a diameter of 90 mm and a thickness of 5 mm waspositioned, and a 0.90 wt % sodium chloride aqueous solution (salinesolution) was poured on the petri dish. Here, the saline solution waspoured until the water level of the saline solution became horizontal tothe upper side of the glass filter. And, one filter paper with adiameter of 90 mm was put thereon.

Subsequently, the above prepared apparatus was mounted on the filterpaper so that the superabsorbent polymer in the apparatus was swollen bythe saline solution under load. After 1 hour, the weight W₄ (g) of theapparatus including swollen superabsorbent polymer was measured.

Using the measured weights, absorbency under load was calculatedaccording to the following Calculation Formula 3.AUL(g/g)=[W ₄(g)−W ₃(g)]/W ₀(g)  [Calculation Formula 3]

In the Formula 3,

W₀ (g) is the initial weight (g) of superabsorbent polymer,

W₃ (g) is the sum of the weight of superabsorbent polymer and the weightof the apparatus capable of giving load to the superabsorbent polymer,and

W₄ (g) is the sum of the weight of superabsorbent polymer and the weightof the apparatus capable of giving load to the superabsorbent polymer,after a saline solution is absorbed in the superabsorbent polymer underload (0.7 psi) for 1 hour.

TABLE 1 Comparative Example Example 1 2 3 4 1 2 3 Before exposure tohigh CRC [g/g] 46.3 44.7 42.3 38.2 35.4 33.7 35.7 temperature AUL [g/g]26.1 25.3 24.3 26.4 24.1 24.6 23.4 P₁ = CRC + 72.4 70.0 66.6 64.6 59.558.3 59.1 AUL After exposure to high CRC [g/g] 46.0 45.8 39.6 36.4 31.030.5 31.2 temperature AUL [g/g] 24.2 24.7 23.1 24.9 22.1 22.8 21.9 P₂ =CRC + 70.2 70.5 62.7 61.3 53.1 53.3 53.1 AUL Y = {[P₁ − P₂]/P₁} * 1003.0 −0.7 5.9 5.1 10.8 8.6 10.2

The absorbencies under load (AUL) of 0.9 psi of the superabsorbentpolymers prepared in Examples 1, 2 and 4 to a saline solution wereadditionally measured. The absorbency under load of 0.9 psi ofsuperabsorbent polymer to a saline solution was measured by the samemethod as the measurement method of absorbency under load of 0.7 psi asexplained above, except using a piston capable of uniformly giving aload of 6.3 kPa (0.9 PSI). As the result of measurement, the absorbencyunder load of 0.9 psi of superabsorbent polymer prepared in Example 1 toa saline solution was 19.4 g/g, the absorbency under load of 0.9 psi ofsuperabsorbent polymer prepared in Example 2 to a saline solution was18.7 g/g, and the absorbency under load of 0.9 psi of superabsorbentpolymer prepared in Example 4 to a saline solution was 25.5 g/g.

Referring to Table 1, it was confirmed that the superabsorbent polymersof Examples 1 to 4 wherein superabsorbent polymers were prepared using aheat-degradable internal crosslinking agent in the content of 80 wt %,based on the total weight of internal crosslinking agent, maintainedexcellent absorption properties, even if exposed to high temperature.

To the contrary, the superabsorbent polymer of Comparative Example 1wherein a heat-degradable internal crosslinking agent was not used, andthe superabsorbent polymers of Comparative Examples 2 and 4 wherein aheat-degradable internal crosslinking agent was used in the content lessthan 80 wt %, based on the total weight of the internal crosslinkingagent, exhibited high change rate of absorption property at hightemperature. And, the superabsorbent polymers of Comparative Examples 1to 3 failed to exhibit both excellent centrifuge retention capacity andabsorbency under load.

Meanwhile, comparing Examples 1 to 4, it is confirmed that among thecompounds represented by the Chemical Formula 1, the compound wherein R¹is methane-1,1-diyl (Example 3); propane-1,3-diyl (Example 1); orpropane-1,2-diyl (Example 2) can realize excellent centrifuge retentioncapacity, and among them, the compound wherein R¹ is propane-1,3-diyl(Example 1); or propane-1,2-diyl (Example 2) can realize more excellentheat stability, centrifuge retention capacity and absorbency under load.

What is claimed is:
 1. A superabsorbent polymer comprising: a base resinpowder comprising crosslinked polymer formed by the crosslinkingpolymerization in an aqueous solution of water soluble ethylenicallyunsaturated monomers having acid groups of which at least a part areneutralized, an inorganic material, an internal crosslinking agentcomprising a compound represented by the following Chemical Formula 1 inthe content of 80 wt % or more, based on the total weight of theinternal crosslinking agent; and a surface crosslink layer on the baseresin powder, formed by the additional crosslinking of the crosslinkedpolymer, wherein the inorganic material is laponite, wherein absorptionproperty change rate at high temperature (Y) calculated by the followingCalculation Formula 1 is −8% to 8%:

in the Chemical Formula 1, R¹ is a divalent organic group derived fromC1-10 alkane, and R² is hydrogen or a methyl group, wherein R¹ in theChemical Formula 1, is methane-1,1-diyl, propane-1,3-diyl,propane-1,2-diyl, propane-1,1-diyl, n-butane-1,4-diyl,n-butane-1,3-diyl, n-butane-1,2-diyl, n-butane-1,1-diyl,2-methylpropane-1,3-diyl, 2-methylpropane-1,2-diyl,2-methylpropane-1,1-diyl, 2-methylbutane-1,4-diyl,2-methylbutane-2,4-diyl, 2-methylbutane-3,4-diyl,2-metylbutane-4,4-diyl, 2-methylbutane-1,3-diyl,2-methylbutane-1,2-diyl, 2-methylbutane-1,1-diyl or2-methylbutane-2,3-diyl,Y={[P ₁ −P ₂]/P ₁}*100  [Calculation Formula 1] in the CalculationFormula 1, Y is absorption property change rate at high temperature, P₁the sum of centrifuge retention capacity (CRC) of the superabsorbentpolymer to a saline solution, and absorbency under load (AUL) of 0.7 psiof the superabsorbent polymer to a saline solution, measured beforeexposed to high temperature, P₂ is the sum of CRC and AUL of thesuperabsorbent polymer, measured after leaving the superabsorbentpolymer in an oven of 185° C. for 60 minutes, and the P₂ is 62 to 85g/g.
 2. The superabsorbent polymer according to claim 1, wherein the P₁is 60 to 85 g/g.
 3. The superabsorbent polymer according to claim 1,wherein R¹ in the Chemical Formula 1, is propane-1,3-diyl orpropane-1,2-diyl.
 4. A superabsorbent polymer comprising: a base resinpowder comprising crosslinked polymer formed by the crosslinkingpolymerization in an aqueous solution of water soluble ethylenicallyunsaturated monomers having acid groups of which at least a part areneutralized, an inorganic material, an internal crosslinking agentcomprising a compound represented by the following Chemical Formula 1 inthe content of 80 wt % or more, based on the total weight of theinternal crosslinking agent; and a surface crosslink layer on the baseresin powder, formed by the additional crosslinking of the crosslinkedpolymer, wherein the inorganic material is laponite, wherein absorptionproperty change rate at high temperature (Y) calculated by the followingCalculation Formula 1 is −8% to 8%, and P₁ in the Calculation Formula 1is 66 to 85 g/g:

in the Chemical Formula 1, R¹ is a divalent organic group derived fromC1-10 alkane, and R² is hydrogen or a methyl group,Y={[P ₁ −P ₂]/P ₁}*100  [Calculation Formula 1] in the CalculationFormula 1, Y is absorption property change rate at high temperature, P₁the sum of centrifuge retention capacity (CRC) of the superabsorbentpolymer to a saline solution, and absorbency under load (AUL) of 0.7 psiof the superabsorbent polymer to a saline solution, measured beforeexposed to high temperature, and P₂ is the sum of CRC and AUL of thesuperabsorbent polymer, measured after leaving the superabsorbentpolymer in an oven of 185° C. for 60 minutes.